The present invention relates to the head driving technique of a liquid jetting apparatus wherein current for driving a driving element provided corresponding to a nozzle for jetting a liquid droplet is reduced in a head of the liquid jetting apparatus.
Heretofore, for an output device of a computer, an ink jet color printer of a type that ink in some colors is jetted from a recording head has been popularized and is widely used to print an image processed by a computer and others in multiple colors and multiple gradations.
For example, in an ink jet printer using a piezoelectric element for a driving element for jetting ink, an ink droplet is jetted from a nozzle by dynamic pressure by each piezoelectric element by selectively driving plural piezoelectric elements respectively provided corresponding to plural nozzles of a print head, a dot is formed on printing paper by making the ink droplet adhere onto the printing paper and printing is performed.
Each piezoelectric element is provided corresponding to a nozzle for jetting an ink droplet, is driven by a driving signal supplied from driver IC (a driving waveform generating circuit) mounted in the print head and makes an ink droplet jetted.
FIG. 3 shows such a head driving device. As shown in FIG. 3, a head driving device 1 is composed of a piezoelectric element 2 respectively provided corresponding to plural nozzles of the ink jet printer, the driving waveform generating circuit 3 for supplying a driving signal to one electrode 2a of each piezoelectric element 2, a current amplifying circuit 4 and a switching circuit 5 respectively provided between the driving waveform generating circuit 3 and each piezoelectric element 2.
FIG. 3 shows only one piezoelectric element 2, however, actually, plural nozzles are provided to a head of an ink jet printer and one piezoelectric element is provided to each nozzle.
A driving signal COM from the driving waveform generating circuit 3 is actually sequentially output to each piezoelectric element 2 via a shift register and others.
The piezoelectric element 2 is formed so that it is displaced by voltage applied between both electrodes 2a, 2b. 
The piezoelectric element 2 is normally charged so that it is substantially at intermediate potential and is formed so that an ink droplet is jetted from a nozzle because pressure is applied to ink in the corresponding nozzle when the piezoelectric element discharges according to a driving signal COM from the driving waveform generating circuit 3.
The driving waveform generating circuit 3 generates a driving signal COM to be supplied to the head of the ink jet printer and is arranged in the body of the printer or a printer head for example.
The current amplifying circuit 4 is composed of two transistors 4a, 4b. Of them, for the first transistor 4a, the collector is connected to a constant voltage power source, the base is connected to one output terminal of the driving waveform generating circuit 3 and the emitter is connected to the input side of the switching circuit 5. Hereby, the first transistor conducts according to a signal from the driving waveform generating circuit 3 and supplies constant voltage Vcc to the piezoelectric element 2 via the switching circuit 5.
For the second transistor 4b, the emitter is connected to the input side of the switching circuit 5, the base is connected to a second output terminal of the driving waveform generating circuit 3 and the collector is grounded. Hereby, the second transistor conducts according to a signal from the driving waveform generating circuit 3 and makes the piezoelectric element 2 discharge via the switching circuit 5.
The switching circuit 5 is turned on at the driving timing of the corresponding piezoelectric element 2 when a control signal is input and outputs a driving signal COM to the piezoelectric element 2.
The switching circuit 5 is actually formed as a so-called transmission gate for turning on or off each piezoelectric element 2.
In the head driving device 1, the current amplifying circuit 4 drives all piezoelectric elements 2 connected to the current amplifying circuit by a pair of transistors 4a, 4b. 
Therefore, as such a transistor that generates maximum current (for example, 30 A) as can supply current required when all piezoelectric elements 2 are simultaneously driven is required to be used for the transistors 4a, 4b, the transistor requiring a lot of power is required and there is a limit in accelerating.
In the meantime, it is also conceivable that for the two transistors 4a, 4b forming the current amplifying circuit 4, plural transistors the respective maximum current of which is relatively small are connected by so-called Darlington connection, however, such Darlington connection of transistors has a problem that even if a load is light, waveform distortion is increased